New Advances in Understanding Inhibition of Myeloperoxidase and Neutrophil Serine Proteases by Two Families of Staphylococcal Innate Immune Evasion Proteins

Neutrophils are the most abundant leukocytes in humans and play an important early role in the innate immune response against microorganisms. Neutrophil phagosomes contain high concentrations of antibacterial enzymes, including myeloperoxidase (MPO) and the neutrophil serine proteases (NSPs). These antibacterial enzymes can also be released extracellularly upon degranulation or as a component of neutrophil extracellular traps (NETs). Due to host/pathogen coevolution, S. aureus expresses a diverse arsenal of innate immune evasion proteins that target many aspects of the neutrophil antibacterial response. In the last decade, two new classes of staphylococcal innate immune evasion proteins that act as potent, selective inhibitors of MPO and NSPs, respectively, have been discovered. The Staphylococcal Peroxidase INhibitor (SPIN) is a small ∼8.3 kDa α-helical bundle protein that blocks MPO activity by interfering with substrate and product exchange with the MPO active site. The Extracellular Adherence Protein (EAP) family consists of three unique proteins comprised of one or more copies of an ∼11 kDa β-grasp domain capable of high-affinity, selective, non-covalent inhibition of NSPs. This brief review article summarizes recent advances in understanding the structural and functional properties of SPIN and EAP family members and outlines some potential avenues for future investigation of these enzyme inhibitors.